Electronically tunable antenna

ABSTRACT

A broadband microwave antenna in the form of a pair of aligned dipoles between which is interposed a ferrite element, such as a sphere of gyromagnetic material, controllably biased so as to vary its point of gyromagnetic resonance. By selectively changing the strength of the bias applied to this ferrite element, the antenna may be accurately tuned over a wide band of frequencies. The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

20 RF. T0 1 RECEIVER Q Q United States Patent [111 3,569,972

[72] Inventor William J- McEvoy 3,128,439 4/1964 Brown et a1 333/242 Ventura, Calif. 3,295,137 12/1966 Fenwick et al 343/787 [21] App]. No. 743,734 3,302,208 1/1967 Hendrickson 343/787 [22] Filed July 10,1968 3,411,111 11/1968 Meyers 343/787X 4s Patented Mar. 9,1911 H 4 3,299,376 1/1967 Blau etal... 333/24.1x [73] Assignee The United States of America as represented 3,480,888 1 1/ 1969 Elliott 333/73 by the secretary Navy Primary Examiner-Eli Lieberman Assistant Examiner-T. Vezeau 5 4] ELECTRONIC ALLY TUNABLE ANTENNA Attorneys-Q. Baxter Warner, George J. Rubens and Howard 3 Claims, 6 Drawing Figs. Murray [52] US. (l 343/747, 343/750, 343/787, 343/822, 343/861, 333/24.1,

333/73 ABSTRACT: A broadband microwave antenna in the form of [51] Ill!- Cl. H0lq 9/16, a pair of aligned dipules between which is interposed a ferrite H lq H q element, such as a sphere of gyromagnetic material, control- [50] Field of Search 343/701, [ably biased so as to vary its point of gyromagnetic resonance. By selectively changing the strength of the bias applied to this I 333/73 ferrite element, the antenna may be accurately tuned over a wide band of frequencies. [56] References cited The invention described herein may be manufactured and UNITED STATES PATENTS used by or for the Government of the United States of Amer- 2,558,487 6/1951 Hills 343/747 ice f r g vernm ntal p rpo i h u h payme f any 2,636,122 4/1953 Ha es 343/747 N roy l i th r n or therefor.

GYROMAGNETIC MEMBER TO SOURCE OF YIG DRIVER CURRENT PIIIEIIIEIIIIIII 9|97| 3569872 2 GYROMAGNETIC R.F. TO MEMBER RECEIVER 22 TO SOURCE OF we DRIVER CURRENT CAPACITIVE OR INDUCTIVE DIPOLE ELEMENT we BIAS MAGNETS 2 IO 136" I4 I I,* I

we SPHERE 26AS r RESONATOR FOR I I DIPOLE ELEMENTS \WG 2 r we SPHERE 2O 22 32 FOR I IMPEDANCE RF TO MATCHING RECEIVER we DRIVER CURRENT -23 SOURCE INVENTOR. WILLIAM J. MC EVOY BY X I-465M Q/i i ATTORNEY m 58 3 E l m I 2 l S 54- Q a --4 F I g. 3

TUNING CURRENT VERSUS FREQUENCY FOR we TUNED ANTENNA mg m:

52 0- j o g l 2 l- PATENTEUMAR 9|97| 3569372 sum 2 OF 3 DIPOLE WITH YIG PATENTED m 9|97l SHEU 3 OF 3 WITHOUT YIG FILTER Fig. 5

WITHYIG FILTER ELECTRONIEAILILY TUNAELE ANTENNA BACKGROUND OF THE INVENTION When space restrictions are imposed on microwave antennas, their efficiency is subject to severe degradation, especially if they are required to operate over a wide band of frequencies. This drawback is particularly serious in aircraft installations, where the physical size of the antenna is frequently limited to less than half an electrical wavelength. Such a parameter is inadequate, and forces a compromise among the factors of antenna gain, field pattern, aperture, and squint. Furthermore, in the case of direction-finding antennas of limited aperture, their fixed-tuning characteristic results in varying parameters as the antenna elements go in and out of resonance with changes in frequency.

SUMMARY or THE INVENTION A feature of the present concept resides in the elimination (or minimization) of the inductively and capacitively reactive components of a fixed-size microwave antenna in order to improve its performance when used for broadband applications.

in a preferred embodiment, this is achieved by integrating into the antenna a ferrite member possessing gyromagnetic proper ties. Such a ferrite member may, for example, consist of a yttrium-iron-garnet (YIG) filter, this element being placed at the center of, or between, the antenna dipoles. The YIG is' tuned electrically to a frequency either above or below the desired antenna frequency so as to provide the proper amount of inductive or capacitive reactance to resonate the antenna elements. If desired, a multiple-ball YIG may be employed to additionally yield an impedance match to the transmission line. It is contemplated that the YIG element or elements may, if desired, be tuned concurrently with the receiver tuning so that the antenna is always resonant at the particular frequency to which the receiver is tuned at any given instant of time.

One object of the present invention, therefore, is to provide an improved form of microwave antenna which is electronically tunable over a wide band of frequencies.

A further object of the invention is to provide an improved form of microwave antenna which incorporates a tunable filter member possessing gyromagnetic properties.

An additional object of the invention is to provide an improved form of microwave antenna incorporating a yttriumiron-garnet filter, and to further provide for the development of an electromagnetic field surrounding said filter such that the resonant frequency of said filter is a function of field strength.

Other objects, advantages, and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FlG. i is a perspective view of an electronically-tunable antenna designed in accordance with a preferred embodiment of the present invention;

FIG. 2 is a schematic representation of the electrical characteristics of the antenna of FIG. 1;

FIG. 3 is a set of curves illustrating frequency vs. gain for the antenna of FIG. I both with and without the addition of the YIG filter;

FIG. 4 is a curve illustrating the manner in which the resonant frequency of the antenna of FIG. 1 varies as a function of the magnitude of the tuning current applied tothe YIG filter; and

FIG. 5 is a field radiation pattern for the antenna of FIG. 1 without the YIG filter; and

MG. 6 is a field radiation pattern for the antenna of FIG. 1 after the YIG filter has been incorporated therein.

DESCRIPTION OF THE PREFERRED EMBODIMENT A microwave antenna of the type to which the present invention is particularly adaptable is illustrated in FIG. 1 of the drawings. This antenna, generally identified by the reference numeral id, is made up of a pair of aligned dipole elements 12 and M backed up by two angularly disposed planar reflectors l6 and id. The latter members are not necessary to an understanding of applicants invention, and may be omitted if desired. The antenna 10 may be dimensioned to operate over a frequency band extending generally from 2 to 4 KMc, although this particular range is obviously exemplary and will depend upon the environment in which the device is to be utilized. Since the dipoles l2 and 14 are of fixed dimensions, it is apparent that the antenna Ill has but a single frequency at which it is resonant, and, as the receiver (not shown) to which the antenna is connected is swept over the band abovedesignated, the antenna will be periodically out of tune to a degree dependent on the extent of its departure from resonance. Signal degradation will thus result, and the overall efficiency of the receiving system adversely affected.

In accordance with a principal feature of the present concept, means are provided for adding to the antenna 10 a controllable amount of inductive or capacitive reactance so as to resonate the antenna at the particular frequency to which the receiver may be tuned at any given instant of time. In a preferred embodiment, this is accomplished by interposing between the antenna dipoles 12 and M and the transmission line 20 to the receiver (not shown) a ferrite member 22 possessing gyromagnetic properties. This ferrite member 22 may, for example, consist of a YIG (yttrium-iron-garnet) filter which is electroncially tunable by means of a DC. current supplied thereto from a suitable source (not shown) over the conductors 24. FIG. 2 illustrates the electrical relationship between the (I6 22 and the remainder of the antenna assembly.

The YIG filter 22 is of a type now known in the art. It consists of a yttrium-iron-garnet crystal 26 (FIG. 2), which may be in the form of a sphere or disc, positioned in a constant magnetic field provided by the permanent bias magnets 28 plus a variable magnetic field developed by the flow of a DC. current through conductors 24 to a coil 30 within which the crystal 26 is positioned. Tuning of the filter is achieved by varying the magnitude of the DC. current flowing through coil 30, this variation in current magnitude being preferably carried out in any conventional manner as a function of the tuning of the receiver (not shown) to which the output of antenna W is applied over conductors 2ft. The bias magnets 28 serve to tune the filter 22 to the center of its passband, thus minimizing the amount of driver power required in conductors 24 in order to vary such tuning over the entire bandwidth (2 to 4 KMc in the example given). 7

The principle of operation of a ferrite member such as the filter 22 is well understood, and discussion of its properties is set forth in chapter 2 of the Handbook of Microwave Ferrite Materials edited by W. H. von Aulock and published in 1965 by the Academic Press of New York and London. For the purposes of the present invention, the filter 22 is tuned to a frequency above or below the desired antenna frequency in order to provide the correct amount of inductive or capacitive reactance to resonate the antenna dipoles l2 and M.

Also included in the filter 22 is a second ferrite sphere 32 (FIG. 2) which is tuned concurrently with sphere 26. The function of sphere 32 is to match the impedance of antenna l0 to that of the transmission line 26) and thus avoid any output loss of RF energy which would otherwise occur.

As an illustration of the manner in which utilization of the YIG filter 22 in accordance with the present concept improves antenna performance, reference is made to FIG. 3 of the drawings. The two frequency vs. gain comparison curves shown therein bring out that the antenna gain variation without the YIG filter is 18 db., while addition of such filter reduces the variation to 12.5 db.

FIG. 4 sets forth the tuning current vs. frequency curve for the YIG tuned antenna of FIGS. 1 and 2. It will be noted that this curve is fairly linear over the entire frequency range from 2 to 4 KMc, thus simplifying the matter of providing D.C. driver current to the coil 30 over conductors 24.

FIGS. 5 and 6 are field radiation patterns for the antenna 10, FIG. 5 being a representative pattern without the YIG filter 22 and FIG. 6 being a pattern obtained after the filter 22 has been added to the assembly. It will be seen that (at a frequency of 2 KMc) the dipole alone has a gain of 16 db. at l02,while at 90 (the equivalent point) the YIG tuned antenna gain is 36.5 db. The difference is 20.5 db. At 210 on each chart, the YIG tuned antenna shows an improvement of 5.0 db.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

I claim:

1. In a microwave receiving antenna assembly designated to operate over a broad band of frequencies, said antenna assembly including a pair of dipoles of fixed physical dimensions aligned end-to-end in spaced apart linear relationship, the improvement which comprises:

means for maintaining said antenna assembly in resonance throughout the band of frequencies over which it is designed to operate, thereby avoiding the degradation in signal reception which would result from an out-ofresonance condition at frequencies other than the antenna tuned frequency;

said means for maintaining said antenna assembly in resonance over said frequency band including an electrically-tunable ferrite member in the form of an yttriumiron-gamet crystal of spherical configuration possessing gyromagnetic properties disposed intermediate said spaced-apart dipoles and electromagnetically associated therewith;

a source of D.C. current;

means for supplying D.C. current from said source to electrically tune said ferrite member; and

at least one permanent magnet disposed adjacent said YIG crystal, the latter lying in the field of constant intensity developed by said magnet.

2. The combination of claim 1 in which said permanent magnet is located within one of said dipoles.

3. The combination of claim 1 in which the means for supplying D.C. current from said source to electrically tune said ferrite member acts to develop an electromagnetic field of variable intensity around said YIG crystal in addition to the field of constant intensity developed by said permanent magnet. 

1. In a microwave receiving antenna assembly designated to operate over a broad band of frequencies, said antenna assembly including a pair of dipoles of fixed physical dimensions aligned end-to-end in spaced apart linear relationship, the improvement which comprises: means for maintaining said antenna assembly in resonance throughout the band of frequencies over which it is designed to operate, thereby avoiding the degradation in signal reception which would result from an out-of-resonance condition at frequencies other than the antenna tuned frequency; said means for maintaining said antenna assembly in resonance over said frequency band including an electrically-tunable ferrite member in the form of an yttrium-iron-garnet crystal of spherical configuration possessing gyromagnetic properties disposed intermediate said spaced-apart dipoles and electromagnetically associated therewith; a source of D.C. current; means for supplying D.C. current from said source to electrically tune said ferrite member; and at least one permanent magnet disposed adjacent said YIG crystal, the latter lying in the field of constant intensity developed by said magnet.
 2. The combination of claim 1 in which said permanent magnet is located within one of said dipoles.
 3. The combination of claim 1 in which the means for supplying D.C. current from said source to electrically tune said ferrite member acts to develop an electromagnetic field of variable intensity around said YIG crystal in addition to the field of constant intensity developed by said permanent magnet. 